Normal Ultrasound Tied to Lower Aneuploidy Risk.

CHICAGO -- Normal ultrasound findings in fetuses considered to be at high risk for chromosomal abnormalities are associated with a two- to threefold decrease in aneuploidy risk.

But the presence of any major abnormality on ultrasound or certain minor abnormalities significantly increases that risk, Dr. William J. Ott said at the annual meeting of the Central Association of Obstetricians and Gynecologists.

Those are the conclusions drawn from the latest study aimed at fine-tuning the role of ultrasound in screening for chromosomal abnormalities among high-risk fetuses. Further studies are needed to address the value of ultrasound screening in low-risk fetuses, said Dr. Ott of St. John's Mercy Medical Center, St. Louis.

He and his associates correlated data from their perinatal ultrasound and cytogenetics labs for 3,775 patients with singleton pregnancies who were undergoing amniocentesis, mostly for advanced maternal age, at a single institution.

From Jan. 1, 1991, to Sept. 1, 1997, 125 chromosomal abnormalities were found. Down syndrome was the most common finding, with trisomy 18 and 13 being the next two most common abnormalities, he said.

Ultrasound findings that were most predictive of aneuploidy on the basis of multivariate analyses were abnormalities of the central nervous system, heart, face, and neck; increased nuchal fold thickness; and increased bowel echogenicity.

When no abnormalities were seen on ultrasound and all long-bone lengths were normal, the incidence of aneuploidy was only 0.8%, which was similar to the 0.4% rate seen in the entire obstetric population delivered during the same time period at the hospital.